Foamed metal

ABSTRACT

FOAMED METALS HAVING IMPROVED CELLULAR STRUCTURE AND STRENGTH ARE PRODUCED BY MELTING THE METAL, THICKENING THE MOLTEN METAL WITH A GASEOUS VISCOSITY INCREASING AGENT, THINNING THE THICKENED METAL TO A DESIRED VISCOSITY BY HOLDING FOR A PERIOD OF TIME, AND FOAMING THE METAL. IN THE THINNING STEP THE METAL MAY BE HELD UNTIL IT SOLIDIFIES, WHICH FURNISHES A CONVENIENT INTERVAL FOR SHIPPING THE PRETHICKENED METAL TO A NEW LOCATION FOR THE FOAMING STEP.

United States Patent 01 3,669,654 Patented June 13, 1972 Rice 3,669,654 FOAMED METAL Currie B. Berry, Jr., Baton Rouge, La., assiguor to Ethyl Corporation, New York, NY. No Drawing. Filed Oct. 30, 1970, Ser. No. 85,814 Int. Cl. B31d 3/00 US. C]. 75-20 F 13 Claims ABSTRACT OF THE DISCLOSURE Foamed metals having improved cellular structure and strength are produced by melting the metal, thickening the molten metal with a gaseous viscosity increasing agent, thinning the thickened metal to a desired viscosity by holding for a period of time, and foaming the metal. In the thinning step the metal may be held until it solidifies, which furnishes a convenient interval for shipping the prethickened metal to a new location for the foaming step.

BACKGROUND OF THE INVENTION In the production of foamed metal, that is, metal having a plurality of randomly dispersed closed cells throughout a metal matrix, a preferred method is to use a heat decomposable foaming agent to generate the gas to form the cells. This technique is disclosed in US. Pats. 2,751,289; 2,895,819; 2,983,597; 3,300,296; and 3,297,431.

Such prior art foams frequently have cells which are of non-uniform structure or undesirable large size. This problem has been to some extent alleviated by increasing the viscosity of the molten metal with various viscosity increasing agents to aid in the subsequent blowing step. Decreased fluidity, i.e. thickening, of the molten metal enables the foaming operation to be relatively prolonged and the foamed metal to be maintained in its heated, fluid condition Without collapsing for relatively prolonged periods since the trapped bubbles cannot readily escape from the thickened melt.

The use of certain thickening agents also cause the foamed product to be strengthened. However, the amount of agent required for optimum strengthening may be so great as to over-thicken the melted metal and render it difiicult to handle in the subsequent foaming operation. Therefore, the present invention has as a primary objective the solving of this problem with over-thickening. The following description of the invention will demonstrate how this is accomplished.

SUMMARY OF THE INVENTION In accordance with the present invention excessive thickening agent is added to a molten metal or its alloy to produce optimum strength in the foamed product and then the molten metal or its alloy is held for a period of time suflicient to allow it to thin to a desired thickness before the foaming step is conducted.

More specifically, the invention provides a process for foaming a metal or metal alloy by melting the metal or metal alloy, thickening the molten metal or metal alloy, allowing the thickened metal or metal alloy to solidify, remelting the solidified metal or metal alloy, and foaming the remelted metal or metal alloy.

Particularly, the invention concerns a process for foammg a metal or metal alloy by melting the metal or metal alloy, thickening the molten metal or metal alloy, thin mug the thickened metal or metal alloy to a desired viscos1ty by holding for a period of time, and foaming the thined metal or metal alloy.

Even further, the invention relates to a process for foaming a metal or metal alloy by melting the metal or metal alloy, strengthening the molten metal or metal alloy by mixing with an oxygen containing agent, thinning the strengthened metal or metal alloy to a desired viscosity by holding for a period of timer, and foaming the thinned metal or metal alloy.

In accordance with the above described procedure, the present invention provides advantages which more than achieve the above mentioned primary objective of the invention. Thus, not only is the molten metal or its alloy thickened to the proper extent to provide highly uniform cellular structure, but the foamed product is also provided with optimum strength.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Among the metals suitable for use in this invention, such as aluminum, magnesium, titanium, and the like, aluminum is preferred. Aluminum alloys are highly preferred and especially desirable are alloys of about 96 to about percent by weight aluminum with about 4 to about 15 percent by weight magnesium.

The first step in the practice of the process of this invention is to melt the metal or its alloy. This may be achieved with any suitable apparatus well known in the art. It is desirable that such apparatus be susceptible to being maintained under an inert gas purge. Gases such as nitrogen are especially suitable for this purge. If a melting pot is employed, the pot is raised to a temperature well above the temperature at which the metal or its alloy normally becomes a liquid. This facilitates quickly melting the metal or its alloy inasmuch as the temperature is allowed to slowly recede as the metal or its alloy becomes completely liquid. Desirably the temperature of the metal or its alloy is stabilized to a point of about 10 F. to about 50 F. above its melting point in order to insure that it will remain liquid during the remainder of process.

Once the metal or its alloy has been liquified, it is agitated or stirred by means known in the art, e.g. an impeller, propeller, turbine, or the like. Where a turbine is employed, a rotation rate of 100 r.p.m. to 10,000 r.p.m. is suitable although 3,000 r.p.m. to 6,000 r.p.m. is generally preferred. It also appears useful to employ a slower rate of stirring before the foaming agent, described hereinbelow, is added to the molten metal or its alloy.

'Once stirring or agitation of the molten metal or its alloy is well under way, the viscosity-strength increasing agent is added to the molten material. Viscosity-strength increasing agents of this invention include air, oxygen, and carbon dioxide, among which oxygen is preferred. Not all viscosity increasing agents also function as strength increasing agents. For example, nitrogen and argon are thickening agents which do not co-function as strengthening agents. In order to increase strength, the agent must contain oxygen and form oxide particles in situ; thus,

adding preformed metal oxide to the melted metal will not increase strength to any apparent or suflicient degree although such oxide will increase 'viscosity. The mechanism by which the strength of the product is increased through oxide formed in situ is not known.

The physical state of the "viscosity-strength increasing agent is not critical inasmuch as these compounds can be employed as solids, liquids, or gases. Thus, the thickening-strengthening agent may be employed in the physical state which is most convenient which in the case of carbon dioxide may be in solid form and in the case of oxygen in the gaseous form.

n the other hand, it is critical that the viscosity-strength increasing agent be uniformly admixed within the molten metal or its alloy. Thus, it is not enough to treat only the surface of the molten material. The thickening-strengthening agent must be blended uniformly into the molten material by agitation or stirring at the rates above described.

Optimum improvement in strength of the foamed metal product usually requires use of more thickening agent than does optimum increase in viscosity of the molten metal. In fact, optimum improvement in strength usually requiresover-thickening to such an extent that it becomes desirable to thin the melt before the actual foaming step is conducted. However, this invention is not limited to over-thickening but instead more generally extends to a process where thinning is practiced subsequent to any degree of thickening. Accordingly, it is suitable to use any quantity of thickening-strengthening agent which meets the purposes of this invention, this quantity being of course dependent upon the particular thickening agent selected, the particular metal or metal alloy being foamed, process conditions, and the type of apparatus employed for the process. Generally, it is suitable tov use a quantity of thickening-strengthening agent within the range of from about 0.02 pound or less to about 10 pounds or more of agent per 100 pounds of the metal or its alloy and preferable to use from about 0.1 to about 1 pound of agent per 100 pounds of the metal or its alloy. More specifically, it is suitable to use from about 0.02 pound or less to about 1 pound or more of oxygen per 100 pounds of metal or its alloy, preferable to use from about 0.1 to about 0.5 pound of oxygen per 100 pounds of metal or its alloy, and more preferable to use from about 0.2 to about 04 pound of oxygen per 100 pounds of metal or its alloy. -It is suitable to use from about 0.2 pound or less to about 10 pounds or more of air per 100 pounds of metal or its alloy, preferable to use from about 1 to about pounds of air per 100 pounds of metal or its alloy, and more preferable to use from about 2 to abaout 4 pounds of air per 100 pounds of metal or its alloy. It is suitable to use from about 0.05 pound or less to about 4 pounds or more of carbon dioxide per 100 pounds of metal or its alloy, preferable to use from about 0.1 to about 2 pounds of carbon dioxide per 100 pounds of metal or its alloy, and more preferable to use from about 0.3 to about 1 pound of carbon dioxide per 100 pounds of metal or its alloy.

Preferably, the viscosity-strength increasing agent is added to the molten material at a rapid rate. The time period of addition may vary from about 5 seconds or less to about 15 minutes or more and is particularly subject to lengthening where very large quantities of thickening agent are employed.

Generally, the pressure at which the viscosity-strength increasing agent is added to the molten metal or its alloy is not highly significant. Subatmospheric, superatmospheric or ambient pressures can be used although ambient pressure is preferred for reasons of economics. However, high pressure may tend to favorably force a gaseous agent into the molten metal or its alloy while the associated closed vessel retards escape of the gaseous agent.

After the addition of thickening-strengthening agent is complete, the molten metal or its alloy is now ready for thinning since it may have been over-thickened in order to obtain enough strengthening. Thinning simply requires holding the metal or its alloy for a period of time until the desired viscosity is obtained. This time may vary as widely as suits the purposes of this invention, for example from about 10 minutes or less to about 120 hours or more, being dependent upon the type of metal or alloy, the amount and type of thickening-strengthening agent utilized, and process conditions. During the holding period, the metal or its alloy may or may not, as desired, be cooled to solidification, which renders it particularly suitable for storage and/or shipment to a different location to be remelted and foamed. In some manufacturing operations, this is a particularly desirable feature.

After the metal or its alloy has been subjected to thinning, it is brought back up to a certain range of molten temperature by the means described above, provided of course cooling has resulted during the thinning step. Once the metal has been brought to a proper temperature, it is ready for the foaming step. A wide variety of blowing agents can be used in the foaming process of this invention. Broadly, all blowing agents described in the prior art are suitable although some Iblowing agents are better than others. However, whatever the blowing agent, the foams of this invention have smaller, more uniform pore size than foams produced from the same metal substrate which have not been made more viscous by the thickening-strengthening agents above described.

Among the various blowing agents, the metal hydrides are preferred, among which titanium, hafnium, or zirconiurn hydrides, especially the latter, are most preferred. Dihydrides and annealed hydrides of less than stoichiometric composition also can be employed. Generally, the best hydride blowing agents are those which decompose to yield gaseous hydrogen at the temperature of the metal or its alloy which is to be foamed and release hydrogen at a relatively slow rate.

The amount of foaming desired determines the amount of hydride or other blowing agent employed; that is, for a dense foam less blowing agent is used than for a lighter foam. Usually, it is preferred to make foams having a 20 percent density or less or to make foams which weigh 110 more than about 20 percent of the weight per given volume of the unexpanded metal. For such foams it is suitable to employ from about 0.5 to about 2 pounds of hafnium hydride, titanium hydride, or zirconium hydride for each pounds of molten metal or its alloy to be foamed. A preferred range is from about 0.6 to about 1.2 pounds per 100 pounds of molten metal to be foamed.

In the foaming step, a temperature is employed which is above that at which the metal or its alloy to be expanded is molten and above the temperature required to thermally decompose the blowing agent. The temperature, however, must not be so high that the blowing gas is released so fast as to cause foaming at an uncontrollable rate. Thus it is preferred to have the temperature of the molten metal or its alloy comparatively cool. Ideally, a temperature is employed at which the melt is just viscous. Taking all these factors into consideration, a typical aluminum-magnesium alloy is foamed at temperatures within-the range from about 1,l30 F. to about l,250 F. and preferably from about 1,l50 F. to about 1,200 F.

Generally, it is suitable to carry out the foaming process at ambient pressure although greater or lesser pressures can be employed with better results under certain circumstances. Lower pressures can be deleterious since they can encourage evolution of blowing gas outside the confines of the mass to be foamed. Super-atmospheric pressures up to 1,500 p.s.i.g. or higher can be used.

In carrying out the blowing step above described, the foaming agent is preferably admixed with the molten metal or its alloy to be formed by using the agitating or stirring means earlier set forth. In the course of such stirring or agitation, the rate is preferably increased above the initial agitation rate at which time the foaming agent is added. Without exception, the more uniform the mixing, the better the foam. All techniques of mixing known in the art which ensure eflicient mixing of materials and liquids can 'be employed. Preferably the mixing step is performed in as short a time as is feasible to achieve uniform mixing. For best results with a typical mixture of blowing agent and molten metal or its alloy, suflicient mixing achieves homogeneity within about seconds. This time period may require an agitation rate with a stirring device of up to 10,000 r.p.m.

It is generally preferable that the addition of foaming agent be at a lower temperature than the addition of the viscosity increasing agent. Accordingly, it is preferred to cool the viscous metal before adding the foaming agent, providedof course the metal has not already been cooled to the proper temperature during the thinning step. Frequently, the cooling is best carried out in a second vessel, i.e., a vessel other than the hot chamber in which viscosity was increased. The second vessel is preheated to Within plus or minus 50 C., preferably plus or minus 20 C. of the foaming temperature, whereupon the viscous metal or its alloy is added thereto.

Subsequent to the addition of the blowing agent, the molten metal or its alloy is allowed to foam. Foaming may occur within an open or closed mold. The size of closed foaming chambers relative to the quantity of metal or its alloy determines density of the product. Regulation of the mold temperature determines the smoothness and thickness of the skin on the finished article.

Having thus described the invention, the following examples are presented as being illustrative and not limiting of the invention.

EXAMPLE I Two hundred pounds of a molten aluminum alloy containing 7 percent magnesium were charged to an agitated vessel. Eight standard cubic feet of oxygen were bubbled into the melt while the agitator was running. The resulting melt was very thick, having the consistancy of heavy soap lather. The melt was held molten at l,l60 F. for one hours, after which the consistancy had thinned out considerably. Two pounds of zirconium hydride were added while the agitator was running, and the mixture immediately was transferred to a mold. The result was an aluminum foam of 10 pounds per cubic foot density and a tensile strength of 230 pounds per square inch.

If the melt had not been held for one hour, it would have been too thick to transfer. If less oxygen had been added for example 4 standard cubic feet, the strength would have been lower, i.e. 160 pounds per cubic foot. If no oxygen had been added, the foam would have been very coarse and weak.

EXAMPLE II Ten pounds of 7 percent magnesium-aluminum alloy was melted in a 6 inch diameter agitated vessel. Air was stirred into the melt for 20 minutes, after which the melt was very viscous. The melt was allowed to stand at 1,200 F. for /2 hour during which the viscosity was reduced considerably. Air was again agitated into the melt, which made it viscous again. The melt was allowed to stand for 30 minutes then agitated with air again. Thirty grams of zirconium hydride were agitated into the moderately thick melt and a foam of pounds per cubic foot was produced which was exceptionally strong, i.e. 400 pounds per square inch tensile strength.

By allowing the molten aluminum to thin out on standing between additions of air, a high oxide content was achieved without having excessive viscosity. The high oxide content resulted in greater strength. Excessive viscosity restricts both transfer of the foaming mixture to a mold and free flowing during expansion so that the mold is not uniformly filled.

6 EXAMPLE IH Two-hundred pounds of a 7 percent magnesium-aluminum alloy are charged into an agitated vessel. Forty standard cubic feet of air are bubbled into the melt while the agitator is running. The melt is extremely viscous. The alloy is poured into molds and allowed to solidify. The resulting pigs of solid alloy are then shipped to another location, remelted, and foamed in the manner heremabove described. The viscosity decreases to a consistancy which permits optimum operation of the foaming unit, and at the same time the foam is of optimum strength.

EXAMPLE IV Two-hundred and fifty pounds of a 7 percent magnesium-aluminum alloy are charged to an agitated vessel. While the agitator is running, two pounds of Dry Ice are sprinkled into the melt. This amount of Dry Ice is found to result in a foam with greatly improved properties, but the resulting melt is too viscous for proper transfer mto'a mold. The melt is held at 1,180 F. for one hour, after which the melt is foamed by agitating in two pounds of zirconium hydride and immediately transferring the mixture into a mold. The reduced viscosity of the melt resulting from the one hour holding period is such that the transfer is easily accomplished.

What is claimed is:

1. A process for foaming a metal or metal alloy comprrsmg melting the metal or metal alloy, strengthening the molten metal or metal allo by mixing an oxygen containing viscositystrength increasing agent therewith, thinning the strengthened metal or metal alloy to a desired viscosity by holding for a period of time, and mixing a blowing agent with the thinned metal or metal alloy, thereby foaming the thinned metal or metal alloy.

0 2. The process of claim 1, wherein the oxygen containing agent is oxygen, air or carbon dioxide.

3. The process of claim 1, wherein the metal is aluminum and from about 0.02 to about 1 pound of oxygen is added per pound of aluminum.

4. The process of claim 1, wherein the metal alloy is aluminum-magnesium and from about 0.02 to about 1 pound of oxygen is added per pound of aluminum.

5. A process for foaming a metal or metal alloy comprlsing melting the metal or metal alloy, strengthening the molten metal or metal alloy by mixing an oxygen containing viscosity-strength increasing agent therewith, thinning the strengthened metal or metal alloy to a desired viscosity by holding for a period of time, allowing the strengthened metal or metal alloy to solidify, remelting the solidified metal or metal alloy, and mixing a blowing agent with the remelted metal or metal alloy, thereby foaming the remelted metal or metal alloy.

6. The process of claim 5, wherein the solidified metal or metal alloy is transported to a new location before being remelted and foamed.

7. The process of claim 5, wherein the viscosity-strength increasing agent is oxygen, air or carbon dioxide.

8. The process of claim 5, wherein the metal is aluminum or an aluminum-magnesium alloy and from about 0.02 to about 1.0 pound of oxygen is added per pound of aluminum or aluminum-magnesium alloy.

9. A process for foaming a metal or metal alloy comprising melting the metal or metal alloy, stirring or agitati ng the metal or metal alloy, adding an oxygen containing strength increasing agent to the molten metal or alloy while continuing stirring or agitation thereof, thinning the strengthened metal or metal alloy to a desired viscosity by holding for a period of time, and mixing a blowing agent with the thinned metal or metal alloy, thereby foaming the thinned metal or metal alloy.

10. The process of claim 9, wherein the strength increasing agent is oxygen, air or carbon dioxide.

11. The process of claim 9, wherein the metal is aluminum or an aluminum-magnesium alloy and from about 0.02 to about 1.0 pound of oxygen is added per pound of aluminum or aluminum-magnesium alloy.

12. The process of claim 9, wherein the metal or metal alloy is solidified after holding and remelted prior to foammg.

13. The process of claim 12, wherein the solidified metal is transported to a new location before being remelted and foamed.

8 References Cited UNITED STATES PATENTS 1,919,730 7/1933 Koenig et a1 75-20 F 5 3,379,517 4/1968 Graper 75-20 3,214,265 10/1965 Fiedler 7520 F 3,305,902 2/1967 Bjorksten 75-20 F X L. DEWAYNE RUTLEDGE, Primary Examiner 10 J. E. LEGRU, Assistant Examiner 

